Which phenotypic traits are under selection from warm, dry climates in black spruce?

Julie Messier, Sabina Henry, Christina M. Caruso, Nathalie Isabel, Patrick Lenz, Benjamin Marquis, William C. Parker, Isabelle Aubin

• Background and Aims Trees are increasingly at risk of maladaptation to their environment as climates change
rapidly worldwide. Although adaptive evolution through natural selection is a key mechanism by which populations
and species can persist in changing environments, we have limited information regarding the phenotypic traits under
selection in warm and dry environments. We answer the following research questions: (1) What ecophysiological
traits are under selection in warm, dry environments? (2) Does intrapopulation trait integration affect the response
to selection in the warmer, drier site? (3) Is the plastic response of traits under selection adaptive? 

• Methods Using Picea mariana (black spruce) as a case study, we studied 425 trees representing seven
provenances across three 50-year-old common garden trials established along a spatial climate gradient across
eastern Canada. We measured height growth rate as a performance metric, and ten traits that reflect water use,
thermoregulation, structural support and photosynthetic rate. 

• Results All traits were under selection in at least one site, mostly in combination with other traits. For two trait
combinations, the strength of selection gradients significantly increased from the colder, wetter site to the warmer,
drier site: water use efficiency (WUE) with Huber value (HV), and carbon-to-nitrogen ratio (CN) with HV. In the
warmer and drier site, trait–trait correlations among these three traits were largely absent, except for CN:HV in
two provenances. Overall, reaction norms suggest that the plastic response was not aligned with selection for trait
pairs in warm, dry climates.

• Conclusions Results suggest that adaptive evolution in response to climate change in P. mariana may favour
phenotypes with fewer needles that are conservative for water and resource use. In the seven study provenances,
intrapopulation trait integration should minimally impede adaptive evolution, but plastic responses to warmer and
drier conditions may constrain the expression of optimally adapted phenotypes.